Many measurement instruments work by sequentially scanning measurement points that are defined on the object to be measured and performing the measurements at each measurement point, or by sequentially scanning (changing) a physical parameter while performing the measurements. Examples of measurement instruments that perform measurements while scanning the measurement points of a sample being measured include atomic force microscopes, scanning electron microscopes, imaging mass spectrometers and the like. Examples of measurement instruments that scan frequency as a physical parameter while making measurements include spectrum analyzers, network analyzers and the like. Examples of measurement instruments that scan across a wavelength of light as a physical parameter include ultraviolet-visible spectrophotometers, infrared spectrophotometers, fluorophotometers and the like. An example of a measurement instrument that scan m/z as a physical parameter while making measurements is a mass spectrometer.
The description hereinbelow focuses primarily on surface analysis instruments such as atomic force microscopes and scanning electron microscopes which are used for the measurement of surface shapes of samples. With such analytic instruments, a predetermined area on a sample is two-dimensionally scanned, and measurements of a very small area on the sample are repeatedly taken to determine, for example, the distribution of certain measurement results in a predetermined area. For example, with atomic force microscopy (AFM), images representing concavities and convexities of the surface of a sample are obtained by one-dimensionally or two-dimensionally scanning the surface of the sample while feeding back and controlling the distance between a sharply pointed probe needle and a sample's surface so that a signal that depends on the distance is kept constant.
Frequency modulation atomic force microscopy (FM-AFM) is a type of atomic force microscopy where a cantilever—which holds a probe needle that approaches the surface of a sample to an atomic level of distance—is made to vibrate at its mechanical resonant frequency so that changes in resonant frequency (frequency shift) caused by interaction between the probe needle and the sample surface can be detected. Since the frequency shift depends on the distance between the probe needle and the sample surface, the sample's surface is two-dimensionally scanned (e.g., raster scanned) in a plane orthogonal to a direction normal to the sample's surface while keeping the frequency shift constant. In this way, concave/convex images of the sample's surface are obtained. Feedback control is performed in this case to keep the distance between the sample and the probe needle constant. At the same time, feedback control is performed to keep the amplitude of the vibration of the probe needle constant.
Generally speaking, with the aforesaid atomic force microscopy, the measurement time (dwell time) spent at each measurement point of the sample is predetermined, and the time required for moving from one measurement point to the next measurement point is sufficiently shorter than the measurement time at each measurement point so that the overall time spent for the measurements, i.e., the time required for obtaining the convex/concave images of the sample's surface, depends on the number of measurement points. With a feedback control such as the afore-described where the separation distance between the probe needle and the sample is kept constant, some time is required after moving to a measurement point for the separation distance to converge to a certain value as determined by factors such as the response characteristics of the feedback control loop. This means that if the measurement time that is defined for each measurement point is too short, measurement values are acquired before the separation distance had converged to a certain value, and the resulting surface shape images that are obtained become inaccurate. At worst, the probe needle may contact the sample surface, damaging either or both and making continued measurements impossible. To avoid these problems, an amount of time has to be calculated that will allow feedback control on the separation distance to sufficiently stabilize even assuming maximum variations in the concavities and convexities and to set a measurement time for each measurement point that is longer than the calculated stabilization time.
However, if that is done, the measurement time at each of the measurement points generally becomes long, and the time required for performing the measurements at all measurement points becomes very long. The result is poor measurement efficiency and reduced throughput. One application of FM-AFM is Kelvin force microscopy (KFM) which performs measurements while compensating for potential differences. With KFM, in addition to the two afore-described feedback control loops found in ordinary FM-AFM, a feedback control loop is provided to compensate for the potential differences that occur at the probe needle. This third feedback control loop includes a lock-in amplifier for measuring the potential difference, and the time required for its stabilization is longer than that of the other feedback control loops. Because of this, the overall measurement time becomes unavoidably long even when the sample being measured has a surface that is relatively flat and changes in potential difference occur only locally.
As described in Patent Literature 1, a known procedure with previous scanning tunneling microscopes is to adjust the measurement time depending on the concavity and convexity of the sample's surface when performing feedback control to keep a constant separation distance between the probe needle and the sample while scanning the measurement points. However, because of the use of a plurality of feedback control loops in FM-AFM, KFM and the like for controlling factors that cause variations during a scan, a problem with the afore-described previous art is the inability to perform a suitable scan control.
Another problem with the previous art is that control that uses the afore-described conventional art cannot be used with instruments such as scanning electron microscope where scanning is usually performed without using a feedback control.
Patent Literature 1: JP 05-164511 A